Excess secretion of prolactin (PRL) a common cause of female infertility, is usually associated with an anterior pituitary prolactinoma. This study will characterize changes in the physiological mechanisms controlling PRL secretion that occur with tumor formation, using as a model the pituitary prolactinoma. This study will characterize changes in the physiological mechanisms controlling PRL secretion that occur with tumor formation, using as a mode the pituitary tumors induced in Fisher 344 rats by treatment with estrogen. Experimental goals will be to identify changes in lactotroph electrophysiology and cellular secretory behavior during the course of estrogen treatment and tumorous transformation. Other studies with normal pituitaries and pituitary tumors have demonstrated cellular heterogeneity by several criteria. Subpopulations of lactotrophs have been identified by different responses to secretory modulators and on the basis of bihormonal secretion. Estrogen treatment is probable associated with shifts in these groups and tumor formation might involve transformation of a particular subpopulation. In this study sequential reverse hemolytic plaque assays will be used to measure changes, during estrogen treatment, in the number of pituitary cells that secrete both PRL and growth hormone. Quantitative reverse hemolytic plaque assays of secretion by individual cell s will be used to identify subpopulations of lactotrophs on the basis of secretory behavior and to follow the evolution of these subpopulations during estrogen treatment. Conventional microelectrode and patch clamp recordings will be used to identify changes, with estrogen treatment, in ion channels of the lactotroph membrane. Lactotrophs for these studies will be identified in primary cultures of anterior pituitary cells by a reverse hemolytic plaque assay. Ionic mechanisms are clearly involved in the control of PRL release and changes in the density of activity of ion channels are expected with estrogen stimulation.. In particular, basal electrical activity, the responsiveness of electrical mechanisms to hormonal modulation, the density of Cja2+ channels, and the characteristics of K+ channels stimulated by dopamine or by thyrotropin releasing hormone will be determined following short and long term estrogen treatment. The reversibility of these changes following removal of estrogen stimulation will be examined to determine their relevance to tumorous transformation.